This invention relates to bearings for machine tool slides. In the past, either roller bearings or hydrodynamic bearings or a combination of both have been used on the slides of machine tools. Hydrodynamic bearings include a bearing pad and a runner along with means for introducing a lubricant between the pad and runner. Hydrodynamic bearings require relative motion between the bearing pad and runner to generate forces in the lubricant between the pad and runner. At zero speed, the lubricant is squeezed out from between the pad and the runner and direct contact is made between the pad and runner. Thereafter, when an initial starting force is applied to the slide, it will stick due to the relatively high static friction between the pad and runner. To start the slide moving, the starting force must be increased to a level which will overcome the static friction, but since the static friction is greater than the dynamic friction, this results in a jerky start. The jerky start characteristic is particularly troublesome when the slide is approaching its commanded position. If the slide stops somewhat short of the commanded position, the servo system has to build up enough force to overcome the static friction, which causes a jerky start that may carry the slide past the commanded position. In the extreme case, the slide may oscillate about the commanded position, forever trying to reach the commanded position, but always overshooting due to the jerky start characteristic.
Hydrostatic bearings include a bearing pad and a runner with a recess in either the pad or runner. Hydrostatic bearings have also been used in machine tools to lubricate slowly moving parts. In hydrostatic bearings, the lubricant is pumped into the recess between the bearing pad and runner with sufficient pressure to keep the pad and runner separated with no relative motion therebetween. In the past, however, the hydrostatic bearings were continuously operated and were thus wasteful of energy and required an expensive sealing system.
The principal object of this invention is to provide a hydrostatic bearing which is adapted for intermittant use in a machine tool to eliminate jerky starting and stopping of the machine tool slides.
Other objects and advantages of the invention will be apparent from the detailed description herein.